The present invention relates to once-through steam generators.
A once-through steam generator (OTSG) is a heat recovery boiler that generates steam, primarily for use in power generation or for another industrial process. Traditional fossil fuel boilers, including heat recovery steam generators (HRSG), are commonly characterized as having three separate sections of heat transfer tubes, with a hot flue gas passing around such heat transfer tubes to generate steam. First, economizer sections heat condensate water, often close to the boiling point, but the water typically remains in a liquid phase. Second, evaporator sections convert the water heated in the economizer sections into saturated steam. Third, superheater sections then superheat the steam so that it can be used to power a steam turbine generator or used in another industrial process. In these traditional fossil fuel boilers, the evaporator sections use a forced or natural circulation design such that water passes multiple times through the flue gas by means of a steam drum, which also contains equipment used to effectively separate the steam generated from the circulated water flow.
Referring now to FIG. 1, an exemplary OTSG 10 is different from such a drum-type HRSG in that an OTSG has a single tube bundle 20 that spans the height of the OTSG 10, and a steam drum is not required. The heat transfer tubes of the tube bundle 20 are in a horizontal orientation, and the flue gas passes through the OTSG 10 on an upward (vertical) path, with cold feedwater entering at the top of the tube bundle 20 and superheated steam exiting at the bottom of the tube bundle 20. In this manner, the OTSG 10 is well-suited to recover waste heat from a combustion turbine 30, as shown in FIG. 1.
There are several advantages with respect to the use of an OTSG as compared to a drum-type HRSG. Without a steam drum, there are fewer controls, and less instrumentation is required, which allows for simplified operation. Also, because the steam drum walls in an HRSG are prone to fatigue failures that result from rapid temperature change, an OTSG unit can usually start up faster. In other words, without a steam drum, there is not the same need to limit large temperature differentials as compared to typical drum-type HRSG.
At the same time, however, there are disadvantages with respect to the use of an OTSG. For example, during a shutdown, there are no provisions to allow water to remain inside of the tube bundle. Therefore, costly boiler feedwater must be drained from the tube bundle at every shutdown. Subsequent start-ups then require cold feedwater to be introduced into a hot OTSG in order to immediately begin generating steam. This introduction of cold feedwater into hot heat transfer tubes causes large thermal fatigue stresses, dramatically reducing cycle life of the heat transfer tubes in the upper inlet areas. Another problem of traditional OTSG designs is that during rapid transient load changes of the combustion turbine, including a trip or a shutdown, there is potential for large slugs of water to enter the lower superheating section of the OTSG. This can also cause large thermal stresses, which further reduces cycle life in these critical areas.